1. Field of the Invention
The present invention relates to a method of screening for a substance which imparts kokumi, a composition containing a substance which imparts kokumi obtained by the screening method, a method for producing a food, seasoning, or drink imparted with kokumi, and a food or drink imparted with kokumi.
2. Brief Description of the Related Art
The calcium receptor (also called the calcium sensing receptor, CaSR) contains 1078 amino acids, and is classified as in class C of the seven-transmembrane receptors (G protein-coupled receptor; GPCR). The cloning of the gene for the calcium receptor was reported in 1993 (Nature, 1993 Dec. 9; 366(6455):575-80). The calcium receptor is known to cause various cellular responses through elevation of the intracellular calcium levels, etc., when activated with calcium, etc. The sequence of the human calcium receptor gene is registered with GenBank (Accession No. NM_000388), and is well conserved among many animal species.
The calcium receptor may promote or suppress various biological functions. Therefore, therapeutic agents which act as activators or inhibitors of the calcium receptor are appropriately used in the treatment of neurological diseases, hepatic diseases, cardiovascular diseases, digestive system diseases, and other diseases, depending on the pathological conditions. For example, the calcium receptor is able to detect increased levels of blood calcium in the parathyroid, and suppress secretion of the parathyroid hormone (PTH) to correct the blood calcium level. Therefore, reduction of the blood calcium level is an expected effect of administration of a calcium receptor activator. It has been reported that when a calcium receptor activator is used to treat secondary hyperparathyroidism in a hemodialysis patient, the PTH level is reduced without the calcium and phosphorus levels increasing.
Since functional studies of the calcium receptor have been conducted primarily during calcium homeostasis, applications so far typically concern bone metabolic diseases in which calcium regulation is involved. However, through analysis of genetic expression, it is now known that the calcium receptor is widely distributed in living bodies in addition to the parathyroid and kidney tissues (J. Endocrinol., 2000 May, 165(2):173-7 and Eur. J. Pharmacol., 2002 Jul. 5, 447(2-3):271-8), and the possibility that the calcium receptor is involved in many various biological functions and the etiology of many diseases has been proposed. For example, the calcium receptor is thought to be involved in the function of the liver, heart, lung, alimentary canal, lymphocyte, and pancreas. It has been confirmed that the calcium receptor is expressed in a wide range of tissues by analyses based on RT-PCR using RNAs extracted from rat tissues. Therefore, the increased importance of activators and inhibitors of the calcium receptor in various applications is becoming recognized.
Moreover, cations such as gadolinium, basic peptides such as polyarginine, polyamines such as spermine, amino acids such as phenylalanine, and so forth have been reported to be calcium receptor activators (Cell Calcium., 2004 March, 35(3):209-16).
Although many specific calcium receptor activators have been developed as described above, few of these compounds are native to living bodies, and those that are native have very low activities. Therefore, therapeutic agents containing these activators pose serious problems including side effects, permeability, and sufficient activity. For example, although it is known that amino acids act on calcium receptors, their use as calcium receptor activators is difficult due their very weak activity. Moreover, although macromolecules such as polyarginine have been reported to be an activator as described above, the activator function is based on their actions as polyvalent cations, which have irregular structures. That is, peptides having a specific structure are not known to be useful as a calcium receptor activator.
In the field of foodstuffs, substances having specific tastes have been used for many years. In particular, substances having the five basic tastes, namely, sweet, salty, sour, bitter, and umami (a delicious taste) have been widely used as seasonings. Substances which enhance these basic tastes have also been widely used. One taste that does not fall within these five basic tastes is “kokumi”. Kokumi means a taste that is not one of the five basic tastes. Kokumi is a taste that not only enhances the five basic tastes but also enhances the marginal tastes of the basic tastes, such as thickness, growth (mouthfulness), continuity, and harmony. Several methods for imparting kokumi have been reported so far. Substances that have been reported to impart kokumi include glutathione (Japanese Patent No. 1464928), heated products of gelatin and tropomyosin (Japanese Patent Laid-open Publication (KOKAI) No. 10-276709), sulfone group-containing compounds (Japanese Patent Laid-open Publication (KOKAI) No. 8-289760), a peptide containing the Asn-His sequence (WO2004/096836), and so forth.
Although the development of various kokumi-imparting substances has been attempted as described above, and those that have been commercialized have been mainly extracts of natural products, there are presently very few examples of isolation of a pure kokumi component from an extract of natural product, such as glutathione and N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine.
Therefore, the development of highly effective, safe, and inexpensive kokumi-imparting substances is desired, and a convenient and highly sensitive method of screening for a kokumi-imparting substance is needed for that purpose.